This invention relates to a lithium ion battery. More particularly, it relates to a high-performance secondary battery which can have an arbitrary shape, such as a thin shape, and a method for forming the same.
There has been an extraordinary demand for reduction in size and weight of portable electronic equipment, and the realization relies heavily on improvement of battery performance. To meet the demand, development and improvement of batteries from various aspects have been proceeding. Characteristics required of batteries include a high voltage, a high energy density, safety, and freedom of shape design. Of conventional batteries, lithium ion batteries are the most promising secondary batteries for realizing a high voltage and a high energy density and are still under study for further improvements.
A lithium ion battery in current practical use comprises a positive plate prepared by applying to a current collector powder of lithium-cobalt oxide, etc., a negative plate similarly prepared by applying to a current collector powder of a carbonaceous material. In order for these electrodes (i.e., a positive electrode and a negative electrode) to function as a lithium ion battery, there must be an ion conducting layer having no electron conductivity through which lithium ions can migrate between the two electrodes. In general, a separator which is made of a porous film of polyethylene, etc. and is filled with a nonaqueous electrolytic solution is interposed between the electrodes as an ion conducting layer.
As shown in FIG. 7, a rigid case 1 made of metal, etc. is used for holding a positive electrode 3, a negative electrode 5, and a separator 4 containing an electrolytic solution, etc. Without the case 1, it would be difficult to maintain the joined state of the electrodes 3 and 5 and the separator 4, and the joint might be separated, resulting in deterioration of battery characteristics. However, the case 1 makes the battery heavy and hinders free shape designing. Hence, batteries that do not need such a case 1 have now been under study. One of the subjects in developing batteries needing no case 1 is how to join the electrodes 3 and 5 to the separator 4 interposed therebetween and to maintain the joined state without applying external force.
In this connection, U.S. Pat. No. 5,460,904 discloses a method for forming an ion conducting layer sandwiched in between a positive electrode and a negative electrode, which teaches use of a polymer having a plasticizer mixed therein with at least part of the plasticizer being displaced with an electrolytic solution. However, the method disclosed in U.S. Pat. No. 5,460,904 is not favorable as a production method because a treatment with an organic solvent is involved in the formation of the ion conducting layer, which necessitates a step for removing the organic solvent and equipment for the organic solvent removal.
Further, in order to realize a practically useful lithium ion battery of thin type, it is necessary to develop a battery structure and a method of forming the structure which secures high productivity in joining a positive electrode and a negative electrode while assuring sufficient structural strength and safety as a battery.
A first method for forming a lithium ion battery according to the invention is a method for forming a lithium ion battery comprising a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer as constituent elements, the constituent elements being impregnated with an electrolytic solution, which comprises the step of joining the positive electrode and the negative electrode with an adhesive resin containing at least partially a plastic resin being present in parts therebetween and the step of deforming the adhesive resin.
A second method for forming a lithium ion battery according to the invention is the above-described first method for forming a lithium ion battery, wherein the step of deforming the adhesive resin is carried out by applying at least the pressure that causes the plastic resin to undergo plastic deformation.
According to the first and second methods, since the adhesive resin contains at least partially a plastic resin, (i) drying is not necessary every time the positive electrode and the negative electrode are joined, (ii) a holding tool for maintaining the joined state is not required, (iii) the step of deforming the adhesive resin does not need to be done one by one and can be effected all at once, (iv) the production equipment can be simplified, which will lead to a great improvement of productivity, and (v) the contact area of the adhesive resin to the positive and negative electrodes is increased by deforming the plastic resin to enhance the adhesive force so that a completed battery may have a high battery strength enough for practical use.
A third method for forming a lithium ion battery according to the invention is the above-described first method, wherein the plastic resin is a thermoplastic resin. According to this method, since the adhesive resin contains a thermoplastic resin at least partially, the production equipment can be simplified, which will lead to a great improvement in productivity. In case of heat generation due to abnormalities such as a short-circuit, the thermoplastic resin melts to shut off the electric current. Therefore, a highly safe lithium ion battery can be obtained.
A fourth method for forming a lithium ion battery according to the invention is the above-described third method, wherein the step of deforming the adhesive resin is carried out by heating.
A fifth method for forming a lithium ion battery according to the invention is the above-described fourth method, wherein the heating is at or above the temperature at which the thermoplastic resin develops flowability.
According to the fourth and fifth methods, adhesive force develops on the thermoplastic resin""s flowing. It follows not only that the contact area between the resin and the positive and negative electrode surfaces increases but that the resin penetrates into the fine pores on the surfaces to produce an anchoring effect. Therefore, a practically useful lithium ion battery having high adhesive strength can be obtained.
A sixth method for forming a lithium ion battery according to the invention is the above-described third method, wherein the step of deforming the adhesive resin is carried out by applying ultrasonic waves under pressure. According to this method, because the resin deforms efficiently by ultrasonic waves application, it is possible to achieve adhesion even under a low pressure or at a low heating temperature. Further, since only the surface portion of the thermoplastic resin is selectively heated, the adhesion can be achieved efficiently.
A first lithium ion battery according to the invention has a laminate electrode body comprising a positive electrode having a positive electrode active material layer joined to a positive electrode current collector, a negative electrode having a negative electrode active material layer joined to a negative electrode current collector, and an adhesive resin comprising a plastic resin at least partially, the adhesive resin being interposed between the positive electrode and the negative electrode and forming voids so that the positive electrode and the negative electrode connect thereby. According to this battery structure, because the adhesive resin comprises a plastic resin at least partially, the production equipment can be simplified, which will lead to a great improvement in productivity. Further, since the contact area of the adhesive resin to the positive and negative electrodes and a separator has been increased by deforming the plastic resin to enhance the adhesive force, the completed battery has a high battery strength enough for practical use.
A second lithium ion battery according to the invention is the above-described first battery wherein the plastic resin is a thermoplastic resin. Because the adhesive resin contains a thermoplastic resin at least partially, the production equipment can be simplified, which will lead to a great improvement in productivity. Further, the lithium ion battery has high safety because the thermoplastic resin melts to shut off the electric current in case of heat generation due to abnormalities such as a short-circuit.
A third lithium ion battery according to the invention is the above-described first battery, wherein the void area is from 30 to 90% based on the interface between the positive and the negative electrodes. In this embodiment, with the voids formed between the positive and the negative electrodes being filled with an electrolytic solution, the resistance to ion conduction between the positive and negative electrodes is reduced sufficiently, which makes use at a high load rate possible and assures adhesive strength sufficient for practical use.
A fourth lithium ion battery according to the invention is the above-described first battery, wherein the distance between the positive electrode and the negative electrode is 100 xcexcm or smaller. In this embodiment, where the voids formed between the positive and the negative electrodes are filled with an electrolytic solution, the resistance to ion conduction between the positive and negative electrodes is reduced sufficiently, which makes use at a high load rate possible.
A fifth lithium ion battery according to the invention is the above-described first battery which has a plurality of the laminate electrode bodies. This embodiment provides a lithium ion battery which is compact and yet has a stable and high capacity.